Without limiting the scope of the invention, its background is described in connection with oil production. More particularly, the invention describes methods, computer models, and related devices aimed at maintaining the highest possible oil production for an oil well with high gas-to-oil ratio over the lifetime of the oil well.
The most advantageous implementation of the present invention is in wells with high Gas-to-Oil Ratio (GOR) defined as GOR greater than about 100 cubic meters of gas over cubic meters of oil, which is sometimes also referred to in other units as about 600 cubic feet of gas per barrel of oil, which is the same as above. Such oil wells may exhibit high and increasing production of gas accompanied by low and decreasing production of oil. In extreme cases, a gas flow regime may be formed with no oil exiting the oil well altogether—even despite adjustments of the surface choke, including either closing or opening thereof. At some point, the gas flow regime may exhaust the reservoir formation pressure and preclude any further oil production, whereby severely limiting a total oil recovery from a particular oil well and even from a particular reservoir formation.
This invention contains further improvements of my earlier U.S. Pat. Nos. 7,172,020 and 7,753,127, incorporated herein in their respective entireties by reference.
A conventional oil well is illustrated in FIG. 1 and includes an oil reservoir formation 1, which is reached by an oil well casing 6 with perforations 2 allowing oil to enter the internal space of the casing 6. An oil well tube 7 is lowered into the casing 6 and fixed at the bottomhole region by spacers 4 or other suitable means. The oil well tube 7 extends to the surface of the well with an adjustable surface choke 8 being used to control the flow of oil and gas from the oil well tube 7.
Optimization of oil production and increase in ultimate oil recovery from an oil well has been a goal of many innovative methods and devices of the prior art. Generally speaking, the bottomhole behavior of oil mixed with gas (and some other ingredients such as water, etc.) has been described in a series of mathematical equations by Muskat. One specific publication by Muskat is incorporated herein by reference in its entirety and describes the mathematical model of oil reservoir: Muskat M. “The Production Histories of Oil Producing Gas-Drive Reservoirs”, published in the Journal of Applied Physics in March of 1945, p. 147-159.
For illustration purposes, a unidimensional axisymmetric system of Muskat equations with corresponding PVT characteristics of fluid and dependencies of relative permeability Kro, Krg from liquid saturation (So) can be described as follows:
                                          1            r                    ⁢                      ∂                          ∂              r                                ⁢                      (                          r              ⁢                                                          ⁢                                                k                  ro                                                                      μ                    o                                    ⁢                                      B                    o                                                              ⁢                                                ∂                  p                                                  ∂                  r                                                      )                          =                              -            158.064                    ⁢                                          ⁢                      ϕ            k                    ⁢                      ∂                          ∂              t                                ⁢                      (                                          S                o                                            B                o                                      )                                              (        1        )                                                      1            r                    ⁢                                    ∂                              ∂                r                                      ⁡                          [                                                r                  (                                                                          ⁢                                                                                    k                        rg                                                                                              μ                          g                                                ⁢                                                  B                          g                                                                                      +                                                                  Rs                        5.615                                            ⁢                                                                        k                          ro                                                                                                      μ                            o                                                    ⁢                                                      B                            o                                                                                                                                )                                ⁢                                                      ∂                    p                                                        ∂                    r                                                              ]                                      =                              -            158.064                    ⁢                                          ⁢                      ϕ            k                    ⁢                      ∂                          ∂              t                                ⁢                      (                                                            S                  g                                                  B                  g                                            +                                                                    S                    o                                                        B                    o                                                  ⁢                                  Rs                  5.615                                                      )                                                          
where: P—pressure in formation; So—oil saturation in formation; Sg—gas saturation in formation; Rs—solution of gas in oil; Bo—oil formation volume factor; Bg—gas formation volume factor; μo—oil viscosity; μg—gas viscosity; φ—formation porosity; K—formation permeability.
For practical purposes, Vogel had simplified the Muskat equations and adapted them to the calculations of oil producing formations. These equations are known as Vogel model and have subsequently been modified by others. One example of such publication is as follows: Vogel, Inflow Performance Relationships for Solution-Gas Drive Wells, as published in Journal of Petroleum Technology, January 1968, pp. 83-92, incorporated herein in its entirety by reference. Unfortunately, Vogel model does not work well in wells with high gas-to-oil ratio. According to Vogel, the dependency of oil rate production of bottomhole pressure is a constantly diminishing parabolic curve with a production peak at zero value of the bottomhole pressure, see for example FIG. 2 of the above-mentioned article. In other words, the lower the bottomhole pressure, the higher the oil rate production from the formation. This is a gross simplification of the bottomhole processes in the formation. In fact, if the bottomhole pressure falls below saturation pressure in case of high GOR, relative permeability coefficient by oil decreases because of gas saturation increase, which in turn is a result of gas being released from oil. Viscosity of so degassed oil also increases. This leads to a decrease of productivity index of formation. This phenomenon affects the oil production rate more than the increasing depression. As a result, decreasing of the bottomhole pressure below saturation pressure can lead to a decrease in oil production rate, rather than to its increase as predicted by Vogel's model, see FIG. 2. In some extreme cases, reliance on Vogel's model will cause a complete switch in production from oil to gas. There is a need therefore for a method allowing calculating the oil production rate in high GOR wells with better accuracy then that allowed by Vogel's model.
It is also known that producing oil wells with high GOR (Gas-to-Oil Ratio) often lose their stability, and this process is accompanied by a sharp increase in GOR. Any attempts to stop this process by using a surface choke or other surface manipulations usually fail, and the oil well gradually switches into a gas production mode. The physics of this process can be explained as follows: in case when a gas cone covers some holes of a perforated section of the well casing 6, quite often that oil well loses stability. This, in turn, leads to a continuing slow increase of the cone height followed by an increase in the gas stream and a decrease in the oil flow. This process continues until the well is completely switched to a gas mode. Even if a switch to a gas mode does not happen, the instability of the well does not allow efficient control of the bottomhole pressure by using a choke at the surface. Similar detrimental phenomena can occur because of formation of a gas skin effect near the bottom of the well. The physics of the skin effect is described in detail in my U.S. Pat. No. 7,172,020. It also shows that this phenomenon leads to a non-conventional shape of the IPR curve (Inflow Pressure Relationship, i.e. the dependence of well oil flow rate of the bottomhole pressure). A notable feature of this curve is the presence of a certain threshold value of the bottomhole pressure (called “Popt—optimal pressure”), at which the greatest possible oil flow rate from a reservoir can be achieved (FIG. 2).
The need exists therefore for methods and devices for continuously producing oil at a maximum possible rate over the life of the oil well in a stable and predictable manner—including in oil wells in high GOR and even in the presence of gas cone and gas skin effects.